FIG. 1 shows a perspective view of a typical computer server 101 with blade servers 103 disposed therein. FIG. 2 shows a perspective view of a computer server 101 in FIG. 1 where blade servers 103 are removed. As can be seen in FIGS. 1 and 2, a chassis 105 has a plurality of slots 107 for enclosing blade servers 103 inside of the chassis 105. Each blade server 103 is removably attached to each socket 201 disposed on a middle wall 203 of a chassis 105. Further, three fan trays 109 are removably disposed under the slots 107 and inside the chassis 105. Each fan tray 109 has an inlet 111 for an air at the front of the fan tray 109.
FIG. 3 shows a sectional view of computer server 101 along the line A-A of FIG. 1. As can be seen in FIG. 3, a front printed circuit board (PCB) assembly 301 of the blade server 103 is disposed at a middle plane 203 of the chassis 105 via the socket 201 so that the front PCB assembly 301 is removably disposed inside the chassis 105, while the rear PCB assembly 307 is fixedly disposed on the rear surface of the middle plane 203. Further, the fan tray 109 is removably disposed inside the chassis 105 under the front PCB assembly 301. The fan tray 109 has an air inlet 111 at the front plane of the fan tray 109 and also has two fans 309 in a row at the upper plate of the fan tray 109 so that the fans 309 provide an air directly to the front PCB assembly 301 as shown in a direction B in FIG. 3. An air filter 311 is disposed between the front PCB assembly 301 and the fans 309 when the fan trays 109 are inserted in the chassis 105. The air filter 311 filters the air from the fans 309 to the front PCB assembly 301. The middle plane 203 of the chassis 305 has an opening 313 beside the back of the fan tray 109 when the fan tray 109 is inserted in the chassis 105 so that the air from the fan is also directed to the rear PCB assembly 307 through the opening 313. In addition, the front PCB assembly 301 and the rear PCB assembly 307 include some electric parts 303 and 305 respectively.
In operation, most of the air from the inlet 111 is directed to the front PCB assembly 301 by the fan 309 as shown in arrow B in FIG. 3, while some of the air from the fans 309 is directed to the rear PCB assembly 307 by the fans 309 through the opening 313 as shown in arrow D in FIG. 3. The air to the front PCB assembly 301 is filtered by the air filter 311, while the air to the rear PCB assembly 307 is not filtered because the rear PCB assembly 307 is not active components and does not require that the air to the rear PCB 307 is filtered.